The long term goal of this proposal is to develop strategies for the treatment of, and for the prevention of breast cancer. In earlier phases of our research, we developed selective aromatase inhibitors for blockade of estrogen synthesis as a treatment strategy. Recently, we have identified aromatase expression and mRNA in epithelial cells of both normal breast and breast cancers. In this application, we propose to investigate the significance of in situ estrogen production to tumor development and progression, mechanisms involved in aromatase regulation in the breast and the effect of aromatase inhibitors and antiestrogens on these processes. We propose studies to establish whether there is an association between in situ estrogen production and growth response in normal human breast and breast cancer tissues, as indicated by our preliminary studies. Measurement of proliferation ([3H]-thymidine incorporation) of breast tumors in histocultures in response to aromatase substrate (testosterone) will be carried out in the presence/absence of aromatase inhibitors, 4-OHA or CGS 20267. Also, immunostaining of tumor sections for proliferating nuclear antigen (PCNA) a marker of cell proliferation, aromatase and estrogen receptors will be used. Some studies suggest that in situ estrogen synthesis may be the major source of estrogen in the breast. We will measure the extent to which in situ estrogen synthesis contributes to total estrogen concentrations in the normal mammary gland in vivo using the baboon model. Epithelial, myoepithelial and stromal cells will be isolated from normal human breast tissue and from aromatase/proliferative positive tumors. The isolated cells will be characterized for a) aromatase activity b) aromatase expression c) aromatase mRNA. As indicators of their response to in situ estrogen production by androgen aromatization, we will measure a) induction of progesterone receptor d) tumor cell proliferation e) tumor cell invasiveness. We propose to identify promoter-regions of the aromatase gene involved in regulation of aromatase expression in isolated epithelial cells from normal breast and breast cancers. Factors will be determined which regulate aromatase expression in epithelial cells of normal breast and breast cancers. This will include a) stimulation by cAMP analogs, glucocorticoids, IL-6 and other cytokines; b) inhibition by estradiol, TGFbeta, inhibin/activin c) effects of stromal cell interaction in vitro. Finally, we will investigate whether in situ estrogen production by tumor epithelial cells is sufficient to promote their growth in vivo in the nude mouse model and whether the factors identified regulate aromatase expression in vivo.